The invention relates to satellite communications systems, and more particularly, to such systems for communicating between mobile subscribers and at least two ground stations connected to a communications network such as the Public Switched Telephone Network (PSTN) or the Internet.
It is known to communicate between a plurality of mobile terminals and a central ground station via an orbiting relay satellite using multiple transmission and reception beams between the satellite and the mobile terminals, while locating the processing for controlling the multiple beams, known as an adaptive beamformer, at the central station rather than on board the satellite. A considerable reduction in complexity of the orbiting satellite results when practicing this art. The technique relies upon communication of multiple signals between the central ground station and the satellite in such a way that their relative phase and amplitude, i.e. coherency, is preserved. One coherent transmission scheme is to sample each of the multiple signals at or above the Nyquist rate and then to form a-high speed time-multiplex of the sampled signals. Known signals may be included in the time-multiplexed stream for facilitating time and frequency synchronization at the satellite.
The reverse link from satellite to central station also preferably uses a high speed time-multiplex of signals received at the satellite by different elements of a multi-element antenna to preserve relative coherency, thus permitting ground-based beamforming for reception as well as transmission. Relative coherency is preserved by time multiplexing through the use of a first time multiplexer for time-multiplexing the real (Inphase or I) parts of a sampled complex signal waveform and a second time-multiplexer, synchronized with the first time-multiplexer, for time-multiplexing the corresponding imaginary (Quadrature or Q) parts of the sampled complex signal waveform, which technique will be referred to as quadrature time division multiplexing.
Mayfield et al., in U.S. Pat. No. 5,903,549 entitled xe2x80x9cGround based beamforming utilizing synchronized code division multiplexingxe2x80x9d teaches using CDMA feeder link transmission to maintain the desired coherency between antenna array element signals, and the ""549 patent is incorporated by reference herein.
In prior art systems, a single central station (ground station) relays signals through one or more satellites, thereby providing service to a number of mobile terminals. However, using a single ground station is not optimal in certain communication systems. Because one or more satellites provide coverage over a significant geographic area, a single ground station may be geographically distant from the end destination of a mobile terminals user""s call. Thus, the routing of the call from the ground station to the end destination may be subject to long-distance charges. Multiple ground stations arranged with sufficient geographic separation minimizes the maximum toll charges required to complete a given mobile terminal user""s call.
A further disadvantage arises from frequency re-use limitations inherent in single-ground station systems. Multiple ground stations that are sufficiently separated geographically may reuse the same frequency spectrum to communicate with the one or more satellites. This is possible because the satellites can easily distinguish between multiple signals in the same frequency spectrum provided there is adequate spatial diversity between the originating signal sources.
However, there remains a need for a communications system in which multiple ground stations advantageously arranged in a given geographic region and each employing beamforming techniques can cooperatively relay signals through one or more supporting satellites to a plurality of mobile terminal users.
The present invention meets this and other needs by providing methods and apparatus for enabling such a communications system. A communications satellite system according to the present invention uses one or more satellites to relay information between a large number of mobile terminals distributed over one or more service regions and a smaller number of ground stations connected to the PSTN or Internet. Beamforming techniques are used such that each satellite transmits a plurality of transmission beams, thereby increasing the number of users supported by each satellite.
The present invention provides both methods and apparatus that permit multiple ground stations to cooperatively relay signals through individual satellites in a manner complementary to the ground-based beamforming techniques used. In a first embodiment of the present invention, different ground stations have control over all the bandwidth in a subset of the transmission beams. In a second embodiment, different ground stations have control over a different portion of the bandwidth but are allowed to use that portion of the bandwidth in any transmission direction. In a third embodiment all ground stations have control over all bandwidth in any transmission direction, but act to avoid interference by not using the same bandwidth in overlapping beams or neighboring directions.
Digital beamforming provides a way to combine and process a plurality of signals for output to a multi-element antenna array in a manner that results in the antenna array outputting one or more directional beams. Both the direction and signal content for each individual beam may be controlled simply through altering the linear combination of the plurality of signals input to the beamforming apparatus. The input signals to the digital beamformer each comprise a stream of complex values and the beamformer combines these input signal streams by performing a series of matrix operations that results in each antenna element radiating a signal representative of a potentially different vector combination of the input signals. By adjusting the set of coefficients applied to the input signals, the digital beamformer can dynamically change the direction and content for any or all of the beams output by antenna array. Because of the directional nature of the output beams, each beam may reuse the same frequency spectrum. Thus, digital beamforming techniques allow a substantial increase in the number of mobile terminal users that may be supported over a given service area by a given satellite.
In ground-based beamforming, the digital beamformer resides in a ground station, processing its plurality of input signals and outputting a set of vectors that will produce the desired beams when input to an appropriate multi-element antenna array. The output vector set is transmitted to a satellite, and systems on-board the satellite feed these vectors to such an antenna array, thereby producing the desired set of transmission beams for coverage of a given service area. Ground-based digital beamforming simplifies the design of the satellite but introduces complications when more than one ground station, each employing ground-based beamforming techniques, relays signals through the same satellite. As noted, the system of the present invention provides methods and apparatus for multiple beam-forming ground stations to relay information through the same satellite in an advantageous manner.
A communications satellite system according to the present invention relays information between a large number of mobile subscribers distributed over a service region and a smaller number of ground stations connected to the PSTN or Internet. The ground stations receive signals from the PSTN or Internet to be relayed to the mobile subscribers via the satellite. The ground stations encode and modulate the signals and form the signals into array element drive signals using a beamformer. The array element drive signals are then multiplexed and translated to a feeder link uplink frequency for transmission to the satellite. More than one ground station transmits to the satellite in the same feeder link frequency band so that the satellite receives the sum of the overlapping ground station signals. The satellite synchronizes to known signal patterns included in the ground station signals and transmits misalignment information to permit ground stations to align their timing and frequency references.
The satellite receives signals from the ground stations using a feeder link receive antenna and divides the feeder link signals into array drive signals and synchronizing signals. The synchronizing signals are processed to derive the aforementioned misalignment information. The array drive signals are modulated to the satellite-to-mobile (i.e. communications downlink) frequency band (e.g. S-band) and amplified by a matrix of S-band power amplifiers to drive a multi-element transmit array so as to create multiple transmission beams directed to mobile subscribers in different cells of the service region. Different ground stations create beams that are separated in the spatial dimension, the frequency dimension, or the time dimension so as to avoid mutual interference.
According to the invention, all ground stations may transmit to the satellite in the same feeder link spectrum and the satellite thus receives a linearly additive combination of signals from all ground stations. The characteristics of each signal are however chosen and generated at each of the ground stations so that the signals from the different ground stations translate to different S-band beams, different S-band frequencies, or different TDMA timeslots (or any combination of these three differences), thus avoiding interference on the S-band downlink between signals originating from different ground stations and destined for different mobile subscribers.
Conversely, signals are received at the satellite from mobile subscribers using a multiplexed element array antenna and after amplification and filtering, the received signals are multiplexed and frequency-translated to a feeder downlink frequency and transponded to the ground stations. The ground stations receive and demultiplex the multi-element array signals transponded from the satellite and digitize the signals for numerical processing. Numerical processing includes digital channelization to divide the signals into a number of frequency channels by means of digital filtering or Fourier Transformation, and digital beamforming to enhance signals received at the satellite from particular directions corresponding to subscribers lying in cells of the service region and served by a given ground station. The ground stations then decode the subscriber signals they are assigned to handle and couple the signals to the PSTN or Internet.